Using geometry nodes, is it possible to transform vertices, that are in a specific vertex group?
It's possible and really really easy to do. You just need two nodes to make it happen
Demonstration:

Node Setup:

Note that test is the name of the vertex group. Also the vertex group should have weight of 1
(I am using blender 2.93)
To change material_index follow this node group:

Demonstration:

P.s. For it to work, U have to select the target faces(editmode)-&gt; right click-&gt; shade smooth them